The present invention relates to a rewritable optical recording medium for performing information writing/reproducing or information writing/reproducing/erasing by utilizing a change of the optical property of a portion of a recording material layer caused by radiation of a light beam such as a laser beam to the recording material layer, and relates also to an optical recording method using such a rewritable optical recording medium. In particular, the present invention relates to an improvement of an optical recording medium in which a signal having four values at the maximum can be recorded, the erasability, the C/N ratio, etc. can be improved, and information can be written or rewritten at a high speed, and relates also to an improvement of an optical recording method using such an optical recording medium.
Heretofore, an optomagnetic recording medium has been proposed as a rewritable optical recording medium for performing information recording by utilizing a light beam such as a laser beam, or the like, and it has been partly used in practice. That is, this system is a system in which information is written by inverting the direction of magnetization of a recording material layer by application of light energy and magnetic field to the recording material layer, and, on the other hand, a reproduction signal is attained by detecting a difference in Faraday rotation angle or Kerr rotation angle caused depending on the direction of magnetization.
Such a system, however, can be adapted to nothing but a limited field, because there is no practical method for performing rewriting within a range of one sector.
On the other hand, a so-called phase changing type optical recording medium utilizing a phase change between "a crystalline phase" and "an amorphous phase" is in a way of research as another rewritable optical recording mediums. That is, this type system is carried out, as shown in FIG. 19, through the steps of: changing a part of a crystalline state (cr) recording material layer (a) into an amorphous state (am) by rapidly cooling the part after melting the part through radiating a high-power laser beam to the part, in general, to thereby utilize the amorphous state as a recording state; and, on the other hand, as shown in FIG. 20, returning the amorphous state to the crystalline state (cr) by gradually cooling the part after melting the part through radiating a low-power laser beam to utilize the crystalline state as an erasing state.
In this type of system, not only information rewriting can be made within a range of one sector by using two light beams (that is, recorded information is erased through one light beam and then information is recorded through the other light beam) but overwriting can be made by using one light beam (that is, information simultaneous rewriting for performing writing/erasing while selectively switching the power of one beam can be made). Accordingly, this type of system has an advantage in that it can be applied to various fields.
However, although a few materials used as the recording materials adapted to the phase changing type optical recording medium are short in the crystallizing time, most materials are slow in the crystallizing speed (long in the crystallizing time, for example, 100 ns or more). Accordingly, a long time is required for easing recorded information, so that there arises a problem in that the rewriting speed is slow.
On the other hand, to solve the problem, a recording method using a phase change between "a crystalline phase" and "a mixture phase in which crystals are mixed in amorphous" instead of the conventional phase change between "a crystalline phase" and "an amorphous phase" has been proposed in Japanese Patent Unexamined Publication No. Sho-64-72341. That is, the proposed recording method is a method in which the erasing time is effectively shortened by making the "mixture phase" instead of the "amorphous phase" correspond to a recording state.
In the improved method in which the "mixture phase" is made to correspond to a recording state, however, a problem arises in that the erasing time cannot be shortened greatly while it can be shortened slightly compared with the conventional method in which the phase is changed from "an amorphous phase" to "a crystalline phase". Another problem arises in that the erasing time is still long in the case of using a recording material excellent in reservation stability (that is, high in the crystallizing temperature). A further problem arises in that deterioration in the erasability and in the C/N ratio is observed because the "mixture phase" is unstable and, accordingly, the signal level is unstable.
Further, both in the conventional method utilizing the phase change between "a crystalline phase" and "an amorphous phase" and in the improved method utilizing the phase change between "a crystalline phase" and "a mixture phase", a signal having values more than two values cannot be recorded. There arises a problem in that the amount of information recorded in one sheet of the optical recording medium is limited.